AI Trajectory

null

{
  "EnableConfigs": null,
  "FocusSymbols": [],
  "KMSANReasoning": "",
  "NeedsKMSAN": false,
  "Reasoning": "The patch modifies the fallback implementations of copy_mc_user_highpage() and copy_mc_highpage() in include/linux/highmem.h located within the #else branch of '#ifdef copy_mc_to_kernel'. On the target architecture (amd64/x86_64), CONFIG_ARCH_HAS_COPY_MC is unconditionally selected, defining copy_mc_to_kernel in arch/x86/include/asm/uaccess.h. Consequently, the modified #else branch is never compiled into an amd64 kernel, making the changes exclusive to other architectures (such as ARM64, RISC-V, or 32-bit x86).",
  "WorthFuzzing": false
}

null

{
  "PatchDiff": "commit 6f3c2c308cc22c9dcf01de7258311cc156535988\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Mon Oct 26 03:59:49 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/include/linux/highmem.h b/include/linux/highmem.h\nindex d7aac9de1c8a4..f90f0c6204a4e 100644\n--- a/include/linux/highmem.h\n+++ b/include/linux/highmem.h\n@@ -474,12 +474,21 @@ static inline int copy_mc_user_highpage(struct page *to, struct page *from,\n \t\t\t\t\tunsigned long vaddr, struct vm_area_struct *vma)\n {\n \tcopy_user_highpage(to, from, vaddr, vma);\n+\t/*\n+\t * The flag can be set while the copy runs, so check after it;\n+\t * a flag landing later can no longer have corrupted this copy.\n+\t */\n+\tif (folio_contain_hwpoisoned_page(page_folio(from)))\n+\t\treturn PAGE_SIZE;\n \treturn 0;\n }\n \n static inline int copy_mc_highpage(struct page *to, struct page *from)\n {\n \tcopy_highpage(to, from);\n+\t/* Same reasoning as in copy_mc_user_highpage(). */\n+\tif (folio_contain_hwpoisoned_page(page_folio(from)))\n+\t\treturn PAGE_SIZE;\n \treturn 0;\n }\n #endif\n"
}

core

null

{
  "EnableConfigs": null,
  "FocusSymbols": [],
  "Reasoning": "The patch modifies the fallback implementations of copy_mc_user_highpage() and copy_mc_highpage() in include/linux/highmem.h located within the #else branch of '#ifdef copy_mc_to_kernel'. On the target architecture (amd64/x86_64), CONFIG_ARCH_HAS_COPY_MC is unconditionally selected, defining copy_mc_to_kernel in arch/x86/include/asm/uaccess.h. Consequently, the modified #else branch is never compiled into an amd64 kernel, making the changes exclusive to other architectures (such as ARM64, RISC-V, or 32-bit x86).",
  "WorthFuzzing": false
}

You are an expert Linux kernel maintainer and security engineer.
Your job is to review a provided patch series and evaluate whether it warrants fuzzing with syzkaller.

IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in
your workspace. Do NOT rely on internal assumptions. You must actively use your code access
tools to inspect the actual source code, callers, and surrounding context.

================================================================================
1. CORE TRIAGE PHILOSOPHY
================================================================================
The goal of patch fuzzing is to discover crashes, regressions, exposed latent bugs,
and newly triggered assertions introduced by the patch series.

- REACHABILITY IS THE PRIMARY GATE:
  Fuzzing can only discover bugs in code that can actually execute in standard virtualized
  environments (GCE or QEMU, utilizing software-emulated devices like USB gadgets, netdev, tun/tap).
  If the modified code is structurally unreachable (see Section 2), it MUST NOT be fuzzed,
  regardless of whether it adds assertions or complex logic.

- DO NOT BLINDLY TRUST "NO FUNCTIONAL CHANGE" (NFCI) OR "REFACTORING" CLAIMS:
  Patch authors routinely label changes as "cleanups", "refactorings", or state
  "No functional change intended". Do NOT take these claims at face value.
  Code refactorings that rearrange logic, introduce helper functions, or alter state management
  in core subsystems frequently introduce subtle semantic shifts or uncover latent kernel bugs.
  If reachable executable code is modified or refactored, it MUST be fuzzed.

- NEW OR MODIFIED ASSERTIONS IN REACHABLE CODE MUST BE FUZZED:
  When a patch introduces or modifies runtime checks or assertions (e.g., WARN_ON*, VM_WARN_ON*,
  BUG_ON*, lockdep_assert*) in reachable code paths, it enforces new or stricter invariants.
  Even if the author believes the invariant always holds, fuzzing is essential to verify whether
  an unusual sequence of operations can violate it.

================================================================================
2. WHEN TO RETURN WorthFuzzing=false (NEGATIVE CRITERIA)
================================================================================
Return WorthFuzzing=false ONLY IF all modified code falls strictly into one or more of these categories:

- Non-kernel and non-executable changes:
  * Modifications to Documentation/, comments, or spelling fixes.
  * User-space directories, self-tests, samples, or scripts (e.g., tools/, samples/, scripts/, usr/)
    that do not affect the compiled kernel image (vmlinux) or kernel modules.
  * Purely decorative logging (e.g., message strings in pr_err, printk, dev_info) or tracepoints
    that do not alter control flow or data structures.
  * Build system or Kconfig changes that do not alter compiled C logic.
- Structurally unreachable hardware:
  * Vendor-specific PCIe switches, SmartNICs, or GPU drivers (e.g., mlxsw, pds_core, qed,
    ionic, amdgpu) requiring physical ASIC/PCIe cards not emulated in standard QEMU.
- Unreachable execution paths:
  * Driver teardown callbacks (.remove, .shutdown, pci_unregister_driver) executed only during
    physical PCI hot-unplug or manual sysfs driver unbinding.
  * Code paths exclusive to architectures other than the target architecture.

================================================================================
3. WHEN TO RETURN WorthFuzzing=true (POSITIVE CRITERIA)
================================================================================
Return WorthFuzzing=true whenever the patch touches reachable executable code, including:
- Core Subsystems:
  * Any logic modifications in memory management (mm/), synchronization/locking (kernel/locking/),
    BPF, scheduler, core networking, VFS, or syscall handling.
- Refactorings and Code Cleanups:
  * Any restructuring of reachable data structures, helper abstractions, or algorithm flows.
- Runtime Assertions and Defensive Checks:
  * Any introduction or alteration of assertions (WARN_ON*, VM_WARN_ON*, BUG_ON*, etc.) in reachable paths.
- Reachable Drivers and Protocols:
  * Drivers accessible via virtual buses (virtio, USB gadget, loopback, netlink, binder, sockets, etc.).

================================================================================
4. EXTRACTING FocusSymbols (PREVENTING DILUTION)
================================================================================
When WorthFuzzing=true, you must extract specific kernel functions into FocusSymbols to guide the fuzzer:

- AVOID UBIQUITOUS LIFECYCLE HOT-PATHS:
  Do NOT list generic, ubiquitous functions called by almost every program in the corpus
  (including, but not limited to: general memory allocators and deallocators, page fault
  and trap handlers, or core synchronization primitives; this is not an exhaustive list).
  Listing ubiquitous functions causes the fuzzer to classify thousands of unrelated tests as "focused",
  which severely dilutes fuzzing effort away from the actual changes.

- TARGET SPECIFIC FEATURE LOGIC AND ENTRYPOINTS:
  List functions that specifically implement the logic being added or altered, or direct API entrypoints
  for the subsystem feature under review.

- HANDLING STATIC INLINE FUNCTIONS IN HEADERS (.h):
  Compiler-inlined static functions (such as static inlines in mm/*.h or include/linux/*.h) lack
  distinct symbol addresses in vmlinux and cannot be targeted directly by symbol coverage filters.
  If the changes are primarily in static inline helpers, identify non-static, feature-specific caller
  functions in .c files that exercise them (avoiding ubiquitous lifecycle wrappers).

================================================================================
5. IDENTIFYING EnableConfigs
================================================================================
Identify any specific CONFIG_ options required to properly compile and reach the modified code:
- Inspect Kconfig files and #ifdef guards; do not make assumptions.
- Check "depends on" lines in Kconfig to include any non-standard parent subsystem configs needed.
- Strip any 'CONFIG_' prefix (e.g., return "NET_IPV4" instead of "CONFIG_NET_IPV4").
Prefer calling several tools at the same time to save round-trips.


Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.

Target architecture: amd64

For your convenience, here is the diff of the changes:
commit 6f3c2c308cc22c9dcf01de7258311cc156535988
Author: syz-cluster <triage@syzkaller.com>
Date:   Mon Oct 26 03:59:49 2026 +0000

    syz-cluster: applied patch under review

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index d7aac9de1c8a4..f90f0c6204a4e 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -474,12 +474,21 @@ static inline int copy_mc_user_highpage(struct page *to, struct page *from,
 					unsigned long vaddr, struct vm_area_struct *vma)
 {
 	copy_user_highpage(to, from, vaddr, vma);
+	/*
+	 * The flag can be set while the copy runs, so check after it;
+	 * a flag landing later can no longer have corrupted this copy.
+	 */
+	if (folio_contain_hwpoisoned_page(page_folio(from)))
+		return PAGE_SIZE;
 	return 0;
 }
 
 static inline int copy_mc_highpage(struct page *to, struct page *from)
 {
 	copy_highpage(to, from);
+	/* Same reasoning as in copy_mc_user_highpage(). */
+	if (folio_contain_hwpoisoned_page(page_folio(from)))
+		return PAGE_SIZE;
 	return 0;
 }
 #endif

gemini-3.8-flash

null

null

							input: 3138
							output: 36
							thoughts: 109
						

Okay, let's dive into this. Here's my thought process on this commit:

**Analyzing Commit 6f3c2c308cc22c9dcf01de7258311cc156535988**

Alright, I need to see what this specific commit, `6f3c2c308cc22c9dcf01de7258311cc156535988`, did. The initial indication is that it's touched `include/linux/highmem.h`. High memory, ah, that rings some bells. I need to understand the change in context. My first instinct is to zero in on the vicinity of line 474 within that header file. That should give me a good starting point to reconstruct the logic in my head.

Now, let's get the file contents of `include/linux/highmem.h` and use `read-file` to grab the content around line 474. It's time to read and figure out the exact change.

{
  "File": "include/linux/highmem.h",
  "FirstLine": 450,
  "LineCount": 50
}

{
  "Contents": " 450:\t\n 451:\t\treturn ret;\n 452:\t}\n 453:\t\n 454:\tstatic inline int copy_mc_highpage(struct page *to, struct page *from)\n 455:\t{\n 456:\t\tunsigned long ret;\n 457:\t\tchar *vfrom, *vto;\n 458:\t\n 459:\t\tvfrom = kmap_local_page(from);\n 460:\t\tvto = kmap_local_page(to);\n 461:\t\tret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);\n 462:\t\tif (!ret)\n 463:\t\t\tkmsan_copy_page_meta(to, from);\n 464:\t\tkunmap_local(vto);\n 465:\t\tkunmap_local(vfrom);\n 466:\t\n 467:\t\tif (ret)\n 468:\t\t\tmemory_failure_queue(page_to_pfn(from), 0);\n 469:\t\n 470:\t\treturn ret;\n 471:\t}\n 472:\t#else\n 473:\tstatic inline int copy_mc_user_highpage(struct page *to, struct page *from,\n 474:\t\t\t\t\t\tunsigned long vaddr, struct vm_area_struct *vma)\n 475:\t{\n 476:\t\tcopy_user_highpage(to, from, vaddr, vma);\n 477:\t\t/*\n 478:\t\t * The flag can be set while the copy runs, so check after it;\n 479:\t\t * a flag landing later can no longer have corrupted this copy.\n 480:\t\t */\n 481:\t\tif (folio_contain_hwpoisoned_page(page_folio(from)))\n 482:\t\t\treturn PAGE_SIZE;\n 483:\t\treturn 0;\n 484:\t}\n 485:\t\n 486:\tstatic inline int copy_mc_highpage(struct page *to, struct page *from)\n 487:\t{\n 488:\t\tcopy_highpage(to, from);\n 489:\t\t/* Same reasoning as in copy_mc_user_highpage(). */\n 490:\t\tif (folio_contain_hwpoisoned_page(page_folio(from)))\n 491:\t\t\treturn PAGE_SIZE;\n 492:\t\treturn 0;\n 493:\t}\n 494:\t#endif\n 495:\t\n 496:\tstatic inline void memcpy_page(struct page *dst_page, size_t dst_off,\n 497:\t\t\t\t       struct page *src_page, size_t src_off,\n 498:\t\t\t\t       size_t len)\n 499:\t{\n"
}

gemini-3.8-flash

null

null

							input: 4262
							output: 36
							thoughts: 23
						

Okay, here's the summary, rendered as if I'm the one thinking it:

**Diving into highmem.h**

Alright, let's get down to it. I'm going to jump into `include/linux/highmem.h`. Specifically, I want to take a look at what's going on from line 450 onward. I know this file holds a lot of the critical definitions and structures related to how the kernel handles high memory. I expect that anything past that line probably deals with the more intricate details or perhaps newer features related to highmem management. I'm keen to see if it's focusing on any specific architecture extensions, advanced memory allocation strategies, or maybe even NUMA considerations, given that's a common area of work for me. I'm really curious to quickly see what the code is doing.

{
  "File": "include/linux/highmem.h",
  "FirstLine": 420,
  "LineCount": 35
}

{
  "Contents": " 420:\t\tkmsan_copy_page_meta(to, from);\n 421:\t\tkunmap_local(vto);\n 422:\t\tkunmap_local(vfrom);\n 423:\t}\n 424:\t\n 425:\t#endif\n 426:\t\n 427:\t#ifdef copy_mc_to_kernel\n 428:\t/*\n 429:\t * If architecture supports machine check exception handling, define the\n 430:\t * #MC versions of copy_user_highpage and copy_highpage. They copy a memory\n 431:\t * page with #MC in source page (@from) handled, and return the number\n 432:\t * of bytes not copied if there was a #MC, otherwise 0 for success.\n 433:\t */\n 434:\tstatic inline int copy_mc_user_highpage(struct page *to, struct page *from,\n 435:\t\t\t\t\t\tunsigned long vaddr, struct vm_area_struct *vma)\n 436:\t{\n 437:\t\tunsigned long ret;\n 438:\t\tchar *vfrom, *vto;\n 439:\t\n 440:\t\tvfrom = kmap_local_page(from);\n 441:\t\tvto = kmap_local_page(to);\n 442:\t\tret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);\n 443:\t\tif (!ret)\n 444:\t\t\tkmsan_unpoison_memory(page_address(to), PAGE_SIZE);\n 445:\t\tkunmap_local(vto);\n 446:\t\tkunmap_local(vfrom);\n 447:\t\n 448:\t\tif (ret)\n 449:\t\t\tmemory_failure_queue(page_to_pfn(from), 0);\n 450:\t\n 451:\t\treturn ret;\n 452:\t}\n 453:\t\n 454:\tstatic inline int copy_mc_highpage(struct page *to, struct page *from)\n"
}

gemini-3.8-flash

null

null

							input: 5049
							output: 21
							thoughts: 66
						

**Analyzing Conditional Compilation in this Kernel Code**

Alright, let's dive in. I'm seeing this `#ifdef copy_mc_to_kernel` at line 427, and immediately after, an `#else` at line 472. My spider senses are tingling – this is conditional compilation, and I need to figure out the context. My immediate question is: on what architectures is this `copy_mc_to_kernel` macro actually *defined*? I need to understand the control flow here. So, I'll go digging. Time to find where this macro is actually set. I'll need to pinpoint which architectures are using this code path.

{
  "Expression": "copy_mc_to_kernel"
}

{
  "Output": "Documentation/admin-guide/device-mapper/dm-pcache.rst=156=Failure handling\n--\nDocumentation/admin-guide/device-mapper/dm-pcache.rst-159-* *pmem media errors* – all metadata copies are read with\nDocumentation/admin-guide/device-mapper/dm-pcache.rst:160:  ``copy_mc_to_kernel``; an uncorrectable error logs and aborts initialisation.\nDocumentation/admin-guide/device-mapper/dm-pcache.rst-161-* *Cache full* – if no free segment can be found, writes return ``-EBUSY``;\n--\narch/powerpc/include/asm/uaccess.h=441=static inline unsigned long __must_check\narch/powerpc/include/asm/uaccess.h:442:copy_mc_to_kernel(void *to, const void *from, unsigned long size)\narch/powerpc/include/asm/uaccess.h-443-{\n--\narch/powerpc/include/asm/uaccess.h-445-}\narch/powerpc/include/asm/uaccess.h:446:#define copy_mc_to_kernel copy_mc_to_kernel\narch/powerpc/include/asm/uaccess.h-447-\n--\narch/x86/include/asm/uaccess.h=493=unsigned long __must_check\narch/x86/include/asm/uaccess.h:494:copy_mc_to_kernel(void *to, const void *from, unsigned len);\narch/x86/include/asm/uaccess.h:495:#define copy_mc_to_kernel copy_mc_to_kernel\narch/x86/include/asm/uaccess.h-496-\n--\narch/x86/lib/copy_mc.c=45=unsigned long copy_mc_enhanced_fast_string(void *dst, const void *src, unsigned len);\n--\narch/x86/lib/copy_mc.c-47-/**\narch/x86/lib/copy_mc.c:48: * copy_mc_to_kernel - memory copy that handles source exceptions\narch/x86/lib/copy_mc.c-49- *\n--\narch/x86/lib/copy_mc.c-62- */\narch/x86/lib/copy_mc.c:63:unsigned long __must_check copy_mc_to_kernel(void *dst, const void *src, unsigned len)\narch/x86/lib/copy_mc.c-64-{\n--\narch/x86/lib/copy_mc.c-81-}\narch/x86/lib/copy_mc.c:82:EXPORT_SYMBOL_GPL(copy_mc_to_kernel);\narch/x86/lib/copy_mc.c-83-\n--\ndrivers/md/dm-pcache/cache_dev.c=130=static int sb_read(struct pcache_cache_dev *cache_dev, struct pcache_sb *sb)\n--\ndrivers/md/dm-pcache/cache_dev.c-133-\ndrivers/md/dm-pcache/cache_dev.c:134:\tif (copy_mc_to_kernel(sb, sb_addr, sizeof(struct pcache_sb)))\ndrivers/md/dm-pcache/cache_dev.c-135-\t\treturn -EIO;\n--\ndrivers/md/dm-pcache/cache_gc.c=20=static bool need_gc(struct pcache_cache *cache, struct pcache_cache_pos *dirty_tail, struct pcache_cache_pos *key_tail)\n--\ndrivers/md/dm-pcache/cache_gc.c-39-\tto_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, cache_seg_remain(key_tail));\ndrivers/md/dm-pcache/cache_gc.c:40:\tret = copy_mc_to_kernel(kset_onmedia, key_addr, to_copy);\ndrivers/md/dm-pcache/cache_gc.c-41-\tif (ret) {\n--\ndrivers/md/dm-pcache/cache_key.c=773=int cache_replay(struct pcache_cache *cache)\n--\ndrivers/md/dm-pcache/cache_key.c-796-\t\tto_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, cache_seg_remain(pos));\ndrivers/md/dm-pcache/cache_key.c:797:\t\tret = copy_mc_to_kernel(kset_onmedia, cache_pos_addr(pos), to_copy);\ndrivers/md/dm-pcache/cache_key.c-798-\t\tif (ret) {\n--\ndrivers/md/dm-pcache/cache_key.c=869=int cache_verify_dirty_tail(struct pcache_cache *cache)\n--\ndrivers/md/dm-pcache/cache_key.c-883-\t\tto_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, cache_seg_remain(\u0026pos));\ndrivers/md/dm-pcache/cache_key.c:884:\t\tret = copy_mc_to_kernel(kset_onmedia, cache_pos_addr(\u0026pos), to_copy);\ndrivers/md/dm-pcache/cache_key.c-885-\t\tif (ret) {\n--\ndrivers/md/dm-pcache/cache_writeback.c=40=static inline bool is_cache_clean(struct pcache_cache *cache, struct pcache_cache_pos *dirty_tail)\n--\ndrivers/md/dm-pcache/cache_writeback.c-51-\tto_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, cache_seg_remain(dirty_tail));\ndrivers/md/dm-pcache/cache_writeback.c:52:\tret = copy_mc_to_kernel(kset_onmedia, addr, to_copy);\ndrivers/md/dm-pcache/cache_writeback.c-53-\tif (ret) {\n--\ndrivers/md/dm-pcache/pcache_internal.h=78=static inline void __must_check *pcache_meta_find_latest(struct pcache_meta_header *header,\n--\ndrivers/md/dm-pcache/pcache_internal.h-89-\t\tmeta_addr = (void *)header + (i * meta_max_size);\ndrivers/md/dm-pcache/pcache_internal.h:90:\t\tif (copy_mc_to_kernel(meta, meta_addr, meta_size)) {\ndrivers/md/dm-pcache/pcache_internal.h-91-\t\t\tpcache_err(\"hardware memory error when copy meta\");\n--\ndrivers/md/dm-pcache/pcache_internal.h-108-\ndrivers/md/dm-pcache/pcache_internal.h:109:\tif (copy_mc_to_kernel(meta_ret, latest, meta_size)) {\ndrivers/md/dm-pcache/pcache_internal.h-110-\t\tpcache_err(\"hardware memory error\");\n--\ndrivers/md/dm-writecache.c=997=static void writecache_resume(struct dm_target *ti)\n--\ndrivers/md/dm-writecache.c-1032-\ndrivers/md/dm-writecache.c:1033:\tr = copy_mc_to_kernel(\u0026sb_seq_count, \u0026sb(wc)-\u003eseq_count,\ndrivers/md/dm-writecache.c-1034-\t\t\t      sizeof(uint64_t));\n--\ndrivers/md/dm-writecache.c-1050-\t\t}\ndrivers/md/dm-writecache.c:1051:\t\tr = copy_mc_to_kernel(\u0026wme, memory_entry(wc, e),\ndrivers/md/dm-writecache.c-1052-\t\t\t\t      sizeof(struct wc_memory_entry));\n--\ndrivers/md/dm-writecache.c=1248=static void bio_copy_block(struct dm_writecache *wc, struct bio *bio, void *data)\n--\ndrivers/md/dm-writecache.c-1265-\ndrivers/md/dm-writecache.c:1266:\t\t\tr = copy_mc_to_kernel(buf, data, size);\ndrivers/md/dm-writecache.c-1267-\t\t\tflush_dcache_page(bio_page(bio));\n--\ndrivers/md/dm-writecache.c=2221=static int writecache_ctr(struct dm_target *ti, unsigned int argc, char **argv)\n--\ndrivers/md/dm-writecache.c-2574-\ndrivers/md/dm-writecache.c:2575:\tr = copy_mc_to_kernel(\u0026s, sb(wc), sizeof(struct wc_memory_superblock));\ndrivers/md/dm-writecache.c-2576-\tif (r) {\n--\ndrivers/md/dm-writecache.c-2585-\t\t}\ndrivers/md/dm-writecache.c:2586:\t\tr = copy_mc_to_kernel(\u0026s, sb(wc),\ndrivers/md/dm-writecache.c-2587-\t\t\t\t      sizeof(struct wc_memory_superblock));\n--\ndrivers/nvdimm/claim.c=233=static int nsio_rw_bytes(struct nd_namespace_common *ndns,\n--\ndrivers/nvdimm/claim.c-252-\t\t\treturn -EIO;\ndrivers/nvdimm/claim.c:253:\t\tif (copy_mc_to_kernel(buf, nsio-\u003eaddr + offset, size) != 0)\ndrivers/nvdimm/claim.c-254-\t\t\treturn -EIO;\n--\ndrivers/nvdimm/pmem.c=142=static blk_status_t read_pmem(struct page *page, unsigned int off,\n--\ndrivers/nvdimm/pmem.c-151-\t\tchunk = min_t(unsigned int, len, PAGE_SIZE - off);\ndrivers/nvdimm/pmem.c:152:\t\trem = copy_mc_to_kernel(mem + off, pmem_addr, chunk);\ndrivers/nvdimm/pmem.c-153-\t\tkunmap_local(mem);\n--\nfs/coredump.c=1270=static int dump_emit_page(struct coredump_params *cprm, struct page *page)\n--\nfs/coredump.c-1308- */\nfs/coredump.c:1309:#ifdef copy_mc_to_kernel\nfs/coredump.c-1310-\n--\nfs/coredump.c=1313=static struct page *dump_page_copy(struct page *src, struct page *dst)\n--\nfs/coredump.c-1315-\tvoid *buf = kmap_local_page(src);\nfs/coredump.c:1316:\tsize_t left = copy_mc_to_kernel(page_address(dst), buf, PAGE_SIZE);\nfs/coredump.c-1317-\tkunmap_local(buf);\n--\nfs/dax.c=1331=static int dax_iomap_copy_around(loff_t pos, uint64_t length, size_t align_size,\n--\nfs/dax.c-1355-\t\telse\nfs/dax.c:1356:\t\t\tret = copy_mc_to_kernel(daddr, saddr, length);\nfs/dax.c-1357-\t\tgoto out;\n--\nfs/dax.c-1364-\t\telse {\nfs/dax.c:1365:\t\t\tret = copy_mc_to_kernel(daddr, saddr, head_off);\nfs/dax.c-1366-\t\t\tif (ret)\n--\nfs/dax.c-1378-\t\telse {\nfs/dax.c:1379:\t\t\tret = copy_mc_to_kernel(daddr + tail_off,\nfs/dax.c-1380-\t\t\t\t\t\tsaddr + tail_off, tail_len);\n--\nfs/dax.c=1445=static int dax_unshare_iter(struct iomap_iter *iter)\n--\nfs/dax.c-1486-\nfs/dax.c:1487:\tif (copy_mc_to_kernel(daddr, saddr, copy_len) != 0)\nfs/dax.c-1488-\t\tret = -EIO;\n--\ninclude/linux/highmem.h=413=static inline void copy_highpage(struct page *to, struct page *from)\n--\ninclude/linux/highmem.h-426-\ninclude/linux/highmem.h:427:#ifdef copy_mc_to_kernel\ninclude/linux/highmem.h-428-/*\n--\ninclude/linux/highmem.h=434=static inline int copy_mc_user_highpage(struct page *to, struct page *from,\n--\ninclude/linux/highmem.h-441-\tvto = kmap_local_page(to);\ninclude/linux/highmem.h:442:\tret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);\ninclude/linux/highmem.h-443-\tif (!ret)\n--\ninclude/linux/highmem.h=454=static inline int copy_mc_highpage(struct page *to, struct page *from)\n--\ninclude/linux/highmem.h-460-\tvto = kmap_local_page(to);\ninclude/linux/highmem.h:461:\tret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);\ninclude/linux/highmem.h-462-\tif (!ret)\n--\ninclude/linux/uaccess.h=226=copy_to_user(void __user *to, const void *from, unsigned long n)\n--\ninclude/linux/uaccess.h-232-\ninclude/linux/uaccess.h:233:#ifndef copy_mc_to_kernel\ninclude/linux/uaccess.h-234-/*\ninclude/linux/uaccess.h:235: * Without arch opt-in this generic copy_mc_to_kernel() will not handle\ninclude/linux/uaccess.h-236- * #MC (or arch equivalent) during source read.\n--\ninclude/linux/uaccess.h=238=static inline unsigned long __must_check\ninclude/linux/uaccess.h:239:copy_mc_to_kernel(void *dst, const void *src, size_t cnt)\ninclude/linux/uaccess.h-240-{\n--\nlib/iov_iter.c=216=size_t memcpy_to_iter_mc(void *iter_to, size_t progress,\n--\nlib/iov_iter.c-218-{\nlib/iov_iter.c:219:\treturn copy_mc_to_kernel(iter_to, from + progress, len);\nlib/iov_iter.c-220-}\n"
}

gemini-3.8-flash

null

null

							input: 9102
							output: 42
							thoughts: 129
						

**Investigating the `copy_mc_to_kernel` Function**

Okay, hold on a second. I need to take a closer look at something. Specifically, I need to understand how `copy_mc_to_kernel` is defined, and its purpose. Let me immediately open up `arch/x86/include/asm/uaccess.h`. There it is, `unsigned long __must_check copy_mc_to_kernel(void *to, const void *from, unsigned len); #define copy_mc_to_kernel copy_mc_to_kernel`. Hmm, it's defined right there, but is there a conditional compilation? I need to make sure this isn't defined only under a specific architecture or configuration. Is there an `#ifdef` protecting it? I need to check the entire `arch/x86/include/asm/uaccess.h` file to be certain I have a complete understanding. I want to be thorough here.

{
  "File": "arch/x86/include/asm/uaccess.h",
  "FirstLine": 485,
  "LineCount": 25
}

{
  "Contents": " 485:\textern unsigned long\n 486:\tcopy_from_user_nmi(void *to, const void __user *from, unsigned long n);\n 487:\textern __must_check long\n 488:\tstrncpy_from_user(char *dst, const char __user *src, long count);\n 489:\t\n 490:\textern __must_check long strnlen_user(const char __user *str, long n);\n 491:\t\n 492:\t#ifdef CONFIG_ARCH_HAS_COPY_MC\n 493:\tunsigned long __must_check\n 494:\tcopy_mc_to_kernel(void *to, const void *from, unsigned len);\n 495:\t#define copy_mc_to_kernel copy_mc_to_kernel\n 496:\t\n 497:\tunsigned long __must_check\n 498:\tcopy_mc_to_user(void __user *to, const void *from, unsigned len);\n 499:\t#endif\n 500:\t\n 501:\t/*\n 502:\t * movsl can be slow when source and dest are not both 8-byte aligned\n 503:\t */\n 504:\t#ifdef CONFIG_X86_INTEL_USERCOPY\n 505:\textern struct movsl_mask {\n 506:\t\tint mask;\n 507:\t} ____cacheline_aligned_in_smp movsl_mask;\n 508:\t#endif\n 509:\t\n"
}

gemini-3.8-flash

null

null

							input: 9888
							output: 21
							thoughts: 62
						

Here's my thought process, as if I were analyzing this code:

**Investigating a Potential Optimization Path in x86 Memory Copy**

Okay, so I'm looking at `arch/x86/include/asm/uaccess.h` and I spot a preprocessor directive, `#ifdef CONFIG_ARCH_HAS_COPY_MC`. This immediately piques my interest. It looks like the x86 architecture has a specific configuration option, `CONFIG_ARCH_HAS_COPY_MC`, that might enable some kind of memory copy optimization. My brain's already starting to churn through possibilities – is this related to memory controllers, cache optimizations, or something else entirely?

The fact that it's in `uaccess.h` suggests it could be related to efficient user-space to kernel-space or kernel-space to user-space memory copies, which are performance-critical in system calls and other contexts.

Now, I need to understand when and why `CONFIG_ARCH_HAS_COPY_MC` is actually defined. I need to figure out which architectures or kernel configurations might trigger this optimization path. Time to fire up the grep utility and search the entire kernel source tree for occurrences of `ARCH_HAS_COPY_MC`. This should quickly reveal the conditions under which this feature is enabled, allowing me to fully grasp the intended architecture.

{
  "Expression": "ARCH_HAS_COPY_MC"
}

{
  "Output": "arch/powerpc/Kconfig=118=config PPC\n--\narch/powerpc/Kconfig-128-\tselect ARCH_ENABLE_MEMORY_HOTPLUG\narch/powerpc/Kconfig:129:\tselect ARCH_HAS_COPY_MC\t\t\tif PPC64\narch/powerpc/Kconfig-130-\tselect ARCH_HAS_CURRENT_STACK_POINTER\n--\narch/powerpc/include/asm/uaccess.h=435=extern __must_check long strnlen_user(const char __user *str, long n);\narch/powerpc/include/asm/uaccess.h-436-\narch/powerpc/include/asm/uaccess.h:437:#ifdef CONFIG_ARCH_HAS_COPY_MC\narch/powerpc/include/asm/uaccess.h-438-unsigned long __must_check\n--\narch/x86/Kconfig=57=config X86\n--\narch/x86/Kconfig-103-\tselect ARCH_HAS_UACCESS_FLUSHCACHE\tif X86_64\narch/x86/Kconfig:104:\tselect ARCH_HAS_COPY_MC\t\t\tif X86_64\narch/x86/Kconfig-105-\tselect ARCH_HAS_SET_MEMORY\n--\narch/x86/include/asm/mce.h=237=extern int mce_p5_enabled;\narch/x86/include/asm/mce.h-238-\narch/x86/include/asm/mce.h:239:#ifdef CONFIG_ARCH_HAS_COPY_MC\narch/x86/include/asm/mce.h-240-extern void enable_copy_mc_fragile(void);\n--\narch/x86/include/asm/uaccess.h=490=extern __must_check long strnlen_user(const char __user *str, long n);\narch/x86/include/asm/uaccess.h-491-\narch/x86/include/asm/uaccess.h:492:#ifdef CONFIG_ARCH_HAS_COPY_MC\narch/x86/include/asm/uaccess.h-493-unsigned long __must_check\n--\narch/x86/lib/Makefile=34=lib-y += pc-conf-reg.o\narch/x86/lib/Makefile:35:lib-$(CONFIG_ARCH_HAS_COPY_MC) += copy_mc.o copy_mc_64.o\narch/x86/lib/Makefile-36-lib-$(CONFIG_INSTRUCTION_DECODER) += insn.o inat.o insn-eval.o\n--\ndrivers/md/dm-writecache.c=46=do {\t\t\t\t\t\t\t\t\\\n--\ndrivers/md/dm-writecache.c-53-\ndrivers/md/dm-writecache.c:54:#if IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC) \u0026\u0026 defined(DM_WRITECACHE_HAS_PMEM)\ndrivers/md/dm-writecache.c-55-#define DM_WRITECACHE_HANDLE_HARDWARE_ERRORS\n--\ninclude/linux/uio.h=277=size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i);\n--\ninclude/linux/uio.h-281-\ninclude/linux/uio.h:282:#ifdef CONFIG_ARCH_HAS_COPY_MC\ninclude/linux/uio.h-283-size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i);\n--\nlib/Kconfig=539=config ARCH_HAS_UACCESS_FLUSHCACHE\n--\nlib/Kconfig-546-# buffer.\nlib/Kconfig:547:config ARCH_HAS_COPY_MC\nlib/Kconfig-548-\tbool\n--\nlib/iov_iter.c=200=EXPORT_SYMBOL(_copy_to_iter);\nlib/iov_iter.c-201-\nlib/iov_iter.c:202:#ifdef CONFIG_ARCH_HAS_COPY_MC\nlib/iov_iter.c-203-static __always_inline\n--\nlib/iov_iter.c=255=EXPORT_SYMBOL_GPL(_copy_mc_to_iter);\nlib/iov_iter.c:256:#endif /* CONFIG_ARCH_HAS_COPY_MC */\nlib/iov_iter.c-257-\n"
}

gemini-3.8-flash

null

null

							input: 11378
							output: 31
							thoughts: 201
						

**Investigating the `ARCH_HAS_COPY_MC` Macro**

Okay, hold on a second. Let me quickly examine `arch/x86/Kconfig`. Right, the `select ARCH_HAS_COPY_MC if X86_64` line immediately jumps out. My target architecture *is* amd64 (x86_64)! That's important.

Now, if `ARCH_HAS_COPY_MC` is indeed selected for x86_64, which I believe is true given the Kconfig entry, I need to check `arch/x86/include/asm/uaccess.h`. Okay, here it is: the presence of that macro defines `copy_mc_to_kernel`, as well as it being unsigned long and `__must_check`.

But now, I have to be absolutely certain... Does `include/linux/highmem.h` include `<asm/uaccess.h>` or the generic `<linux/uaccess.h>`? I'll just confirm with a quick glance at `include/linux/highmem.h`. This is a critical point; the path of the included header matters for this check.

{
  "Expression": "uaccess",
  "PathPrefixes": [
    "include/linux/highmem.h"
  ]
}

{
  "Output": "include/linux/highmem.h-10-#include \u003clinux/mm.h\u003e\ninclude/linux/highmem.h:11:#include \u003clinux/uaccess.h\u003e\ninclude/linux/highmem.h-12-#include \u003clinux/hardirq.h\u003e\n"
}

gemini-3.8-flash

null

null

							input: 11943
							output: 36
							thoughts: 79
						

Here's my thought process, as requested:

**Navigating the Kernel's Memory Access Macros**

Okay, so I'm digging into how the kernel handles high memory and, specifically, how it interacts with user space. I see that `include/linux/highmem.h` pulls in `<linux/uaccess.h`. That's not surprising; we need those memory access routines. And of course, `<linux/uaccess.h>` itself depends on the architecture-specific stuff in `<asm/uaccess.h>`. Standard stuff for abstraction.

But, hang on a second... I need to refresh my memory on the exact implementation details of those access macros. Let me jump over to `include/linux/uaccess.h`. Specifically, I think the critical definitions are probably somewhere around lines 220 to 250 in that file. That's where I need to focus to understand the intricacies of how user-space memory is accessed. Let's see what's there...

{
  "File": "include/linux/uaccess.h",
  "FirstLine": 220,
  "LineCount": 35
}

{
  "Contents": " 220:\t\tif (!check_copy_size(to, n, false))\n 221:\t\t\treturn n;\n 222:\t\treturn _copy_from_user(to, from, n);\n 223:\t}\n 224:\t\n 225:\tstatic __always_inline unsigned long __must_check\n 226:\tcopy_to_user(void __user *to, const void *from, unsigned long n)\n 227:\t{\n 228:\t\tif (!check_copy_size(from, n, true))\n 229:\t\t\treturn n;\n 230:\t\treturn _copy_to_user(to, from, n);\n 231:\t}\n 232:\t\n 233:\t#ifndef copy_mc_to_kernel\n 234:\t/*\n 235:\t * Without arch opt-in this generic copy_mc_to_kernel() will not handle\n 236:\t * #MC (or arch equivalent) during source read.\n 237:\t */\n 238:\tstatic inline unsigned long __must_check\n 239:\tcopy_mc_to_kernel(void *dst, const void *src, size_t cnt)\n 240:\t{\n 241:\t\tmemcpy(dst, src, cnt);\n 242:\t\treturn 0;\n 243:\t}\n 244:\t#endif\n 245:\t\n 246:\tstatic __always_inline void pagefault_disabled_inc(void)\n 247:\t{\n 248:\t\tcurrent-\u003epagefault_disabled++;\n 249:\t}\n 250:\t\n 251:\tstatic __always_inline void pagefault_disabled_dec(void)\n 252:\t{\n 253:\t\tcurrent-\u003epagefault_disabled--;\n 254:\t}\n"
}

gemini-3.8-flash

null

null

							input: 12763
							output: 35
							thoughts: 788
						

**Investigating the Definition of `copy_mc_to_kernel` on amd64**

Okay, let's dive into this `copy_mc_to_kernel` function. I'm starting by checking `include/linux/uaccess.h`, line 233, and I see it's conditionally defined.  Interesting, it's not simply defined directly.  Aha, but wait, it's not defined there! That's strange.

Now, let me look at `arch/x86/include/asm/uaccess.h`. Okay, here we go... it *is* defined here, as a macro, under the condition of `CONFIG_ARCH_HAS_COPY_MC`.  And wait, on x86_64, `CONFIG_ARCH_HAS_COPY_MC` *is* true, according to `arch/x86/Kconfig` (it's selected if `X86_64` is true!). So on x86_64 it is a macro definition!

Alright, let's keep going. I'll check `include/linux/highmem.h`.  Ah, there it is!  It's being used there, with conditional compilation based on the definition of `copy_mc_to_kernel`.  If defined, we get `copy_mc_user_highpage` and `copy_mc_highpage` defined too.

So, on amd64, is it defined or not? Yes! Defined as the macro.

Now, hold on. Is `CONFIG_ARCH_HAS_COPY_MC` always selected on x86_64? I should verify that.  Checking `arch/x86/Kconfig` again.  Hmm, yep, under the `X86` config, `select ARCH_HAS_COPY_MC if X86_64`. I need to ensure there isn't a case where it's disabled. Can it be disabled? No, it's a `select`.

But wait. It might not be defined on 32-bit x86?  Yes! In 32-bit x86 (`X86_32`), `CONFIG_ARCH_HAS_COPY_MC` is NOT selected!

Wait a second, the prompt says "Target architecture: amd64". Okay, this is important. We have confirmed the target is amd64. Therefore, is it possible for `copy_mc_to_kernel` to NOT be defined on amd64 under ANY config?

Let's do one last check in `arch/x86/Kconfig`, around line 104. Yep, it seems definitive: On amd64, `copy_mc_to_kernel` *is* defined.  I'm confident in that conclusion now.

{
  "File": "arch/x86/Kconfig",
  "FirstLine": 95,
  "LineCount": 25
}

{
  "Contents": "  95:\t\tselect ARCH_HAS_NMI_SAFE_THIS_CPU_OPS\n  96:\t\tselect ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE\n  97:\t\tselect ARCH_HAS_PMEM_API\t\tif X86_64\n  98:\t\tselect ARCH_HAS_PREEMPT_LAZY\n  99:\t\tselect ARCH_HAS_PTDUMP\n 100:\t\tselect ARCH_HAS_PTE_SPECIAL\n 101:\t\tselect ARCH_HAS_HW_PTE_YOUNG\n 102:\t\tselect ARCH_HAS_NONLEAF_PMD_YOUNG\tif PGTABLE_LEVELS \u003e 2\n 103:\t\tselect ARCH_HAS_UACCESS_FLUSHCACHE\tif X86_64\n 104:\t\tselect ARCH_HAS_COPY_MC\t\t\tif X86_64\n 105:\t\tselect ARCH_HAS_SET_MEMORY\n 106:\t\tselect ARCH_HAS_SET_DIRECT_MAP\n 107:\t\tselect ARCH_HAS_STRICT_KERNEL_RWX\n 108:\t\tselect ARCH_HAS_STRICT_MODULE_RWX\n 109:\t\tselect ARCH_HAS_SYNC_CORE_BEFORE_USERMODE\n 110:\t\tselect ARCH_HAS_SYSCALL_WRAPPER\n 111:\t\tselect ARCH_HAS_UBSAN\n 112:\t\tselect ARCH_HAS_CHECK_WX\n 113:\t\tselect ARCH_HAS_ZONE_DMA_SET if EXPERT\n 114:\t\tselect ARCH_HAVE_NMI_SAFE_CMPXCHG\n 115:\t\tselect ARCH_HAVE_EXTRA_ELF_NOTES\n 116:\t\tselect ARCH_MEMORY_ORDER_TSO\n 117:\t\tselect ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE\n 118:\t\tselect ARCH_MIGHT_HAVE_ACPI_PDC\t\tif ACPI\n 119:\t\tselect ARCH_MIGHT_HAVE_PC_PARPORT\n"
}

gemini-3.8-flash

null

null

							input: 14702
							output: 34
							thoughts: 1945
						

**Unraveling a Potential Unreachable Code Path**

Okay, so I'm looking at this patch and immediately, my spidey-sense is tingling. It's modifying the `#else` branch within `include/linux/highmem.h`, specifically concerning `copy_mc_user_highpage` and `copy_mc_highpage`. The critical piece here is the `#ifdef copy_mc_to_kernel` preprocessor directive.

First things first: on an x86_64 system (which is what I'm focused on), I know that `CONFIG_ARCH_HAS_COPY_MC` is *always* selected. In `arch/x86/include/asm/uaccess.h`, we see that if `CONFIG_ARCH_HAS_COPY_MC` is defined then `copy_mc_to_kernel` is defined as a macro.

The critical question now becomes: is the `#else` branch *ever* executed on amd64? Could `copy_mc_to_kernel` *not* be defined when the `#ifdef` in `highmem.h` is evaluated? The code path is defined within `include/linux/uaccess.h`, and that file is *always* included prior to where `copy_mc_to_kernel` is used in `highmem.h`.

I then recall my knowledge of where `copy_mc_to_kernel` *is* defined and start listing the architectures that it impacts: `x86_64` and `powerpc64` define `copy_mc_to_kernel` as a macro. Other architectures such as `arm64`, `riscv`, `s390`, and `mips` do *not* define `copy_mc_to_kernel` as a macro. But, `include/linux/uaccess.h` provides a generic inline `copy_mc_to_kernel` function, which is *not* a macro definition.

This is the key realization. The `#ifdef` in `highmem.h` is checking for a preprocessor macro, not just *a* definition of the function. This means the `#else` branch would be triggered on architectures that *don't* define `copy_mc_to_kernel` as a macro, which is, at least, `arm64`, `riscv`, `s390`, `mips` and `x86_32`.

Given that I'm targeting x86_64, the `#else` branch here is unreachable. This code path is, therefore, exclusive to architectures other than amd64.

{
  "File": "include/linux/highmem.h",
  "FirstLine": 1,
  "LineCount": 30
}

{
  "Contents": "   1:\t/* SPDX-License-Identifier: GPL-2.0 */\n   2:\t#ifndef _LINUX_HIGHMEM_H\n   3:\t#define _LINUX_HIGHMEM_H\n   4:\t\n   5:\t#include \u003clinux/fs.h\u003e\n   6:\t#include \u003clinux/kernel.h\u003e\n   7:\t#include \u003clinux/bug.h\u003e\n   8:\t#include \u003clinux/cacheflush.h\u003e\n   9:\t#include \u003clinux/kmsan.h\u003e\n  10:\t#include \u003clinux/mm.h\u003e\n  11:\t#include \u003clinux/uaccess.h\u003e\n  12:\t#include \u003clinux/hardirq.h\u003e\n  13:\t\n  14:\t#include \"highmem-internal.h\"\n  15:\t\n  16:\t/**\n  17:\t * kmap - Map a page for long term usage\n  18:\t * @page:\tPointer to the page to be mapped\n  19:\t *\n  20:\t * Returns: The virtual address of the mapping\n  21:\t *\n  22:\t * Can only be invoked from preemptible task context because on 32bit\n  23:\t * systems with CONFIG_HIGHMEM enabled this function might sleep.\n  24:\t *\n  25:\t * For systems with CONFIG_HIGHMEM=n and for pages in the low memory area\n  26:\t * this returns the virtual address of the direct kernel mapping.\n  27:\t *\n  28:\t * The returned virtual address is globally visible and valid up to the\n  29:\t * point where it is unmapped via kunmap(). The pointer can be handed to\n  30:\t * other contexts.\n"
}

gemini-3.8-flash

null

null

							input: 17679
							output: 34
							thoughts: 14
						

Here's a summary of my thought process:

**Diving into uaccess.h**

Okay, so I need to understand how user-space memory access is handled within the kernel. My first instinct is to go directly to the source, so I'm thinking, "Let's check `include/linux/uaccess.h`". That file is the canonical location for those critical definitions and functions. It's the starting point for exploring the kernel's mechanisms for safely and securely interacting with memory that belongs to user-space processes. I expect to find the macros, inline functions, and type definitions that form the core of the user-kernel boundary for memory operations. This is where the magic happens, and I want to see precisely how it's defined and implemented.

{
  "File": "include/linux/uaccess.h",
  "FirstLine": 1,
  "LineCount": 30
}

{
  "Contents": "   1:\t/* SPDX-License-Identifier: GPL-2.0 */\n   2:\t#ifndef __LINUX_UACCESS_H__\n   3:\t#define __LINUX_UACCESS_H__\n   4:\t\n   5:\t#include \u003clinux/cleanup.h\u003e\n   6:\t#include \u003clinux/fault-inject-usercopy.h\u003e\n   7:\t#include \u003clinux/instrumented.h\u003e\n   8:\t#include \u003clinux/minmax.h\u003e\n   9:\t#include \u003clinux/nospec.h\u003e\n  10:\t#include \u003clinux/sched.h\u003e\n  11:\t#include \u003clinux/ucopysize.h\u003e\n  12:\t\n  13:\t#include \u003casm/uaccess.h\u003e\n  14:\t\n  15:\t/*\n  16:\t * Architectures that support memory tagging (assigning tags to memory regions,\n  17:\t * embedding these tags into addresses that point to these memory regions, and\n  18:\t * checking that the memory and the pointer tags match on memory accesses)\n  19:\t * redefine this macro to strip tags from pointers.\n  20:\t *\n  21:\t * Passing down mm_struct allows to define untagging rules on per-process\n  22:\t * basis.\n  23:\t *\n  24:\t * It's defined as noop for architectures that don't support memory tagging.\n  25:\t */\n  26:\t#ifndef untagged_addr\n  27:\t#define untagged_addr(addr) (addr)\n  28:\t#endif\n  29:\t\n  30:\t#ifndef untagged_addr_remote\n"
}

gemini-3.8-flash

null

null

							input: 18319
							output: 22
							thoughts: 535
						

**My Analysis of `copy_mc_to_kernel` on amd64**

Okay, let's break this down. I'm looking at how `copy_mc_to_kernel` is handled, and I'm focusing specifically on the amd64 architecture. My initial thought is to trace the definition of `copy_mc_to_kernel`, which starts with `include/linux/uaccess.h` including `<asm/uaccess.h>`.  Within the x86-specific include path (`arch/x86/include/asm/uaccess.h`), I see `copy_mc_to_kernel` is conditionally defined via `#ifdef CONFIG_ARCH_HAS_COPY_MC`.  Aha! The plot thickens.

The presence of `copy_mc_to_kernel` hinges on `CONFIG_ARCH_HAS_COPY_MC`, which is selected, apparently, only if `X86_64`. My initial reaction is, is this always true? Under amd64, are there *any* scenarios where `CONFIG_ARCH_HAS_COPY_MC` might *not* be set, or where `copy_mc_to_kernel` might remain undefined? I need to verify this because conditional definitions are always a source of headaches later.

So, I check `lib/Kconfig` and `arch/x86/Kconfig`. `ARCH_HAS_COPY_MC` is a boolean in `lib/Kconfig`. Diving into `arch/x86/Kconfig`, I find that `X86` is unconditionally defined and that `ARCH_HAS_COPY_MC` is directly `select`ed *if* `X86_64`. And, since the target architecture is amd64, this means that amd64 is `ARCH=x86` with `CONFIG_64BIT=y` and `CONFIG_X86_64=y`.

Therefore, the critical link is: `ARCH_HAS_COPY_MC` is *always* 1 on amd64.  `#ifdef CONFIG_ARCH_HAS_COPY_MC` is always true within `arch/x86/include/asm/uaccess.h`.  This means that `#define copy_mc_to_kernel copy_mc_to_kernel` is *always* defined on amd64! This is good.

Now, my mind shifts to `include/linux/highmem.h`. I recall seeing that `copy_mc_to_kernel` is conditionally used in highmem.h.  Based on the above, the `#ifdef copy_mc_to_kernel` branch *is always* taken on amd64.  The `#else` branch, is therefore, *never* compiled on amd64. This is a crucial observation.

**Hold on, before I proceed with any conclusions, let's verify this thoroughly!** I'm not comfortable with the assumption that this is *always* true without deeper scrutiny. I want to understand *why* this conditional exists in the first place. What's the historical context? What was the rationale behind this? Let's check commit logs and history. I will run `git log` on `include/linux/highmem.h` to see what changes have been made to this file and specifically how `copy_mc_to_kernel` has been used. This historical context is critical for a full and proper understanding of why the current code is the way it is. This is a very important next step!

{
  "Expression": "copy_mc_user_highpage"
}

{
  "Output": "include/linux/highmem.h=413=static inline void copy_highpage(struct page *to, struct page *from)\n--\ninclude/linux/highmem.h-433- */\ninclude/linux/highmem.h:434:static inline int copy_mc_user_highpage(struct page *to, struct page *from,\ninclude/linux/highmem.h-435-\t\t\t\t\tunsigned long vaddr, struct vm_area_struct *vma)\n--\ninclude/linux/highmem.h=454=static inline int copy_mc_highpage(struct page *to, struct page *from)\n--\ninclude/linux/highmem.h-472-#else\ninclude/linux/highmem.h:473:static inline int copy_mc_user_highpage(struct page *to, struct page *from,\ninclude/linux/highmem.h-474-\t\t\t\t\tunsigned long vaddr, struct vm_area_struct *vma)\n--\ninclude/linux/highmem.h=486=static inline int copy_mc_highpage(struct page *to, struct page *from)\n--\ninclude/linux/highmem.h-488-\tcopy_highpage(to, from);\ninclude/linux/highmem.h:489:\t/* Same reasoning as in copy_mc_user_highpage(). */\ninclude/linux/highmem.h-490-\tif (folio_contain_hwpoisoned_page(page_folio(from)))\n--\nmm/khugepaged.c=854=static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *folio,\n--\nmm/khugepaged.c-876-\t\tsrc_page = pte_page(pteval);\nmm/khugepaged.c:877:\t\tif (copy_mc_user_highpage(page, src_page, src_addr, vma) \u003e 0) {\nmm/khugepaged.c-878-\t\t\tresult = SCAN_COPY_MC;\n--\nmm/ksm.c=3143=struct folio *ksm_might_need_to_copy(struct folio *folio,\n--\nmm/ksm.c-3174-\tif (new_folio) {\nmm/ksm.c:3175:\t\tif (copy_mc_user_highpage(folio_page(new_folio, 0), page,\nmm/ksm.c-3176-\t\t\t\t\t\t\t\taddr, vma)) {\n--\nmm/memory.c=1085=copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,\n--\nmm/memory.c-1100-\nmm/memory.c:1101:\tif (copy_mc_user_highpage(\u0026new_folio-\u003epage, page, addr, src_vma))\nmm/memory.c-1102-\t\treturn -EHWPOISON;\n--\nmm/memory.c=3749=static inline int __wp_page_copy_user(struct page *dst, struct page *src,\n--\nmm/memory.c-3759-\tif (likely(src)) {\nmm/memory.c:3760:\t\tif (copy_mc_user_highpage(dst, src, addr, vma))\nmm/memory.c-3761-\t\t\treturn -EHWPOISON;\n--\nmm/memory.c=6107=static vm_fault_t do_cow_fault(struct vm_fault *vmf)\n--\nmm/memory.c-6130-\nmm/memory.c:6131:\tif (copy_mc_user_highpage(vmf-\u003ecow_page, vmf-\u003epage, vmf-\u003eaddress, vma)) {\nmm/memory.c-6132-\t\tret = VM_FAULT_HWPOISON;\n--\nmm/memory.c=7728=static int copy_user_gigantic_page(struct folio *dst, struct folio *src,\n--\nmm/memory.c-7742-\t\tcond_resched();\nmm/memory.c:7743:\t\tif (copy_mc_user_highpage(dst_page, src_page,\nmm/memory.c-7744-\t\t\t\t\t  addr + i*PAGE_SIZE, vma))\n--\nmm/memory.c=7756=static int copy_subpage(unsigned long addr, int idx, void *arg)\n--\nmm/memory.c-7761-\nmm/memory.c:7762:\tif (copy_mc_user_highpage(dst, src, addr, copy_arg-\u003evma))\nmm/memory.c-7763-\t\treturn -EHWPOISON;\n"
}

gemini-3.8-flash

null

null

							input: 20814
							output: 23
							thoughts: 22
						

Okay, let's dive into this. Here's how I'd approach this task, as if these are my immediate thoughts:

**Investigating `folio_contain_hwpoisoned_page`**

Alright, my first instinct is to get a handle on this `folio_contain_hwpoisoned_page` function or macro. It's obviously significant, given the name. The goal here is to understand its role in this system. I need to figure out how it's defined and, crucially, where it's being *used*. Function definitions always give clues but usage reveals true significance. Is it a central component, or some obscure utility? I need to understand its relationship to the surrounding code. My immediate thought is to use my tools to trace its definition and see where the code calls it. Once I've looked at those calling points, I can get a better idea of what it does.

{
  "Expression": "folio_contain_hwpoisoned_page"
}

{
  "Output": "include/linux/highmem.h=473=static inline int copy_mc_user_highpage(struct page *to, struct page *from,\n--\ninclude/linux/highmem.h-480-\t */\ninclude/linux/highmem.h:481:\tif (folio_contain_hwpoisoned_page(page_folio(from)))\ninclude/linux/highmem.h-482-\t\treturn PAGE_SIZE;\n--\ninclude/linux/highmem.h=486=static inline int copy_mc_highpage(struct page *to, struct page *from)\n--\ninclude/linux/highmem.h-489-\t/* Same reasoning as in copy_mc_user_highpage(). */\ninclude/linux/highmem.h:490:\tif (folio_contain_hwpoisoned_page(page_folio(from)))\ninclude/linux/highmem.h-491-\t\treturn PAGE_SIZE;\n--\ninclude/linux/page-flags.h=1067=static inline bool is_page_hwpoison(const struct page *page)\n--\ninclude/linux/page-flags.h-1076-\ninclude/linux/page-flags.h:1077:static inline bool folio_contain_hwpoisoned_page(struct folio *folio)\ninclude/linux/page-flags.h-1078-{\n--\nmm/huge_memory.c=4614=static bool thp_underused(struct folio *folio)\n--\nmm/huge_memory.c-4621-\nmm/huge_memory.c:4622:\tif (folio_contain_hwpoisoned_page(folio))\nmm/huge_memory.c-4623-\t\treturn false;\n--\nmm/memory_hotplug.c=1874=static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)\n--\nmm/memory_hotplug.c-1896-\nmm/memory_hotplug.c:1897:\t\tif (folio_contain_hwpoisoned_page(folio)) {\nmm/memory_hotplug.c-1898-\t\t\t/*\n--\nmm/shmem.c=3447=shmem_write_begin(const struct kiocb *iocb, struct address_space *mapping,\n--\nmm/shmem.c-3473-\nmm/shmem.c:3474:\tif (folio_contain_hwpoisoned_page(folio)) {\nmm/shmem.c-3475-\t\tfolio_unlock(folio);\n--\nmm/vmscan.c=1161=static unsigned int shrink_folio_list(struct list_head *folio_list,\n--\nmm/vmscan.c-1194-\nmm/vmscan.c:1195:\t\tif (folio_contain_hwpoisoned_page(folio)) {\nmm/vmscan.c-1196-\t\t\t/*\n"
}

gemini-3.8-flash

null

null

							input: 21782
							output: 22
							thoughts: 1571
						

**Analyzing the Conditional Compilation of `copy_mc_to_kernel` in `include/linux/highmem.h`**

Okay, let's break this down. My initial thought was to examine the differences between the `#ifdef copy_mc_to_kernel` and `#else` blocks within `include/linux/highmem.h`, specifically lines 425-495. I see that the `#ifdef` block defines `copy_mc_user_highpage` and `copy_mc_highpage` to use `copy_mc_to_kernel`, handling Machine Check exceptions. The `#else` block provides a fallback using the standard `copy_user_highpage` and `copy_highpage`. The key difference in the `#else` block that this patch introduces is a check using `folio_contain_hwpoisoned_page`.

My immediate question: Is this `#else` block EVER used on an `amd64` system? Let's trace it down.

First, I know that `amd64` builds typically use `ARCH=x86` with `CONFIG_64BIT=y`. I need to understand if the `copy_mc_to_kernel` macro is defined or not.

I went to `arch/x86/Kconfig` and found that `CONFIG_ARCH_HAS_COPY_MC` is selected if `X86_64` is set. Given that this is an `amd64` build, `X86_64` is always set. That means `CONFIG_ARCH_HAS_COPY_MC` is *always* selected.

Next, I looked at `arch/x86/include/asm/uaccess.h`. There, I confirmed that if `CONFIG_ARCH_HAS_COPY_MC` is defined, `copy_mc_to_kernel` is defined. More importantly, in `include/linux/highmem.h`, `<linux/uaccess.h>` is included, thus, `<asm/uaccess.h>` is included.

Because of this, whenever `include/linux/highmem.h` is parsed, `copy_mc_to_kernel` is defined as a macro! Line 427, `#ifdef copy_mc_to_kernel`, is therefore *always* true. The code block following `#else` is therefore never compiled. Thus, on `amd64`, the patch containing the check of `folio_contain_hwpoisoned_page` is never built into the kernel.

Just to be absolutely sure, I'll quickly check for any potential `#undef copy_mc_to_kernel` calls within the codebase, just in case there's some unexpected scenario where the macro *could* be undefined. That should confirm this understanding.

no matches

{
  "Expression": "undef copy_mc_to_kernel"
}

{
  "error": "no matches"
}

gemini-3.8-flash

null

null

							input: 23985
							output: 180
							thoughts: 1447
						

**My Analysis of the Patch and WorthFuzzing Determination**

Okay, let's break this down. First, there's no `#undef copy_mc_to_kernel`, which is the starting point. My initial thought is to make sure it's not defined or undefined anywhere else, specifically within `arch/x86/include/asm/uaccess.h`. Then, I need to understand if `CONFIG_ARCH_HAS_COPY_MC` depends on anything besides `X86_64`. I check `lib/Kconfig` and find it's a simple boolean config.

Digging into `arch/x86/Kconfig`, I see `select ARCH_HAS_COPY_MC if X86_64` under `config X86`. Since `config X86` is `def_bool y`, any 64-bit x86 kernel will have `CONFIG_X86_64=y`, thus unconditionally selecting `ARCH_HAS_COPY_MC`.

Hold on! Could a `.c` file include `<linux/highmem.h>` without `<asm/uaccess.h>` having defined `copy_mc_to_kernel`? I'm thinking about the preprocessor here, line-by-line inclusion.

Looking at `include/linux/highmem.h`, the file includes `<linux/uaccess.h>` right at the top. And, `uaccess.h` includes `<asm/uaccess.h>`. So, by the time the preprocessor gets to line 427 in `highmem.h`, `<asm/uaccess.h>` *has* to have been included.

Alright, time to re-read the negative criteria. It needs to be precise.

The key sentence is: "* Code paths exclusive to architectures other than the target architecture."

And it hits me! This patch is *exactly* that. It modifies the `#else` branch of the `#ifdef copy_mc_to_kernel` block in `include/linux/highmem.h`. On `amd64`, `copy_mc_to_kernel` is always defined because `CONFIG_ARCH_HAS_COPY_MC` is selected by default on `X86_64`. Therefore, on amd64, the `#else` block is never compiled. It's only for architectures without machine check copy support, like ARM64, RISC-V, or 32-bit x86. This modified code is completely exclusive to architectures other than amd64. It won't get compiled into the amd64 kernel.

Let's do a double-check to nail this down:

*   Target architecture: amd64.
*   Modified file: `include/linux/highmem.h`.
*   Lines modified: 481-482 and 490-491.
*   Both are within the `#else` of the `#ifdef copy_mc_to_kernel`.
*   In `arch/x86/include/asm/uaccess.h`: `copy_mc_to_kernel` is defined as a macro if `CONFIG_ARCH_HAS_COPY_MC` is enabled.
*   On amd64, `CONFIG_ARCH_HAS_COPY_MC` *is* unconditionally enabled (`select ARCH_HAS_COPY_MC if X86_64`).
*   Thus, the `#ifdef copy_mc_to_kernel` evaluates to true on amd64, and the `#else` block is discarded.
*   The code path is exclusive to architectures that lack `copy_mc_to_kernel` support.
*   Therefore, `WorthFuzzing` should be `false`.

So here's what I've got:

*   `WorthFuzzing`: false
*   `FocusSymbols`: \[ ]
*   `EnableConfigs`: \[ ]
*   `Reasoning`: Concise explanation: The patch modifies fallback implementations of `copy_mc_user_highpage` and `copy_mc_highpage` in `include/linux/highmem.h` under `#ifndef copy_mc_to_kernel`. On amd64, `CONFIG_ARCH_HAS_COPY_MC` is always selected, defining `copy_mc_to_kernel` as a macro. Thus, the `#else` fallback is never compiled on amd64 and is exclusive to architectures without `copy_mc_to_kernel` support.

I think I've got it covered, so I'll check my tool parameters and make the call.

{
  "EnableConfigs": [],
  "FocusSymbols": [],
  "Reasoning": "The patch modifies the fallback implementations of copy_mc_user_highpage() and copy_mc_highpage() in include/linux/highmem.h located within the #else branch of '#ifdef copy_mc_to_kernel'. On the target architecture (amd64/x86_64), CONFIG_ARCH_HAS_COPY_MC is unconditionally selected, defining copy_mc_to_kernel in arch/x86/include/asm/uaccess.h. Consequently, the modified #else branch is never compiled into an amd64 kernel, making the changes exclusive to other architectures (such as ARM64, RISC-V, or 32-bit x86).",
  "WorthFuzzing": false
}

{
  "EnableConfigs": null,
  "FocusSymbols": [],
  "Reasoning": "The patch modifies the fallback implementations of copy_mc_user_highpage() and copy_mc_highpage() in include/linux/highmem.h located within the #else branch of '#ifdef copy_mc_to_kernel'. On the target architecture (amd64/x86_64), CONFIG_ARCH_HAS_COPY_MC is unconditionally selected, defining copy_mc_to_kernel in arch/x86/include/asm/uaccess.h. Consequently, the modified #else branch is never compiled into an amd64 kernel, making the changes exclusive to other architectures (such as ARM64, RISC-V, or 32-bit x86).",
  "WorthFuzzing": false
}

{
  "WorthFuzzing": false
}

null